In chapter 41 of the publication entitled Polymers for High Technology published in 1987 by the American Chemical Society, in an article entitled "Preparation of Polyimide Mono- and Multilayer Films" (incorporated herein by reference and made a part hereof), the authors Kakimoto et al begin by mentioning that aromatic polyimides have been widely used as insulating materials in microelectronics, recent developments in this field toward higher integration of devices required ultra thin films of polyimides, and the minimum thickness of polyimide films cast by spin coating was about 1 micron. The authors then go on to disclose another type of ultra thin film, namely Langmuir-Blodgett films of polyimide.
Langmuir-Blodgett ("L-B") films are a family of films named after the scientists, Irving Langmuir and Katherine Blodgett, who were, respectively, the first to undertake scientific research on such films and the first to deposit them in multilayer form on substrates. As an initial step in the preparation of a Langmuir-Blodgett film, there is floated on a body of water (or other aqueous medium) a quantity of a substance comprising molecules which are polarized in the sense that these molecules each have a hydrophobic end and an opposite hydrophilic end. A material constituted of molecules of such sort is known as an amphiphilic material.
The substance so floated is allowed to spread on the water's surface until it forms at the air-water interface a monomolecular layer ("monolayer") in the thickness dimension of which the molecules of the amphiphilic material are mostly oriented in the same direction in that the hydrophilic or "head" end of the molecules extend toward the water and the hydrophobic or "tail" end of such molecules extend toward the air. Depending on the density of packing of the molecules in the two dimensions of the interface, such monomolecular Langmuir-Blodgett film can be either in the gaseous, liquid or solid phase.
By techniques well-known in the art, a Langmuir-Blodgett monolayer so formed at such an interface can be transferred to a substrate by drawing the substrate from the air into the water (or conversely) so as to intercept the monolayer and cause its deposit on the substrate while, concurrently, the surface pressure of the floating monolayer is regulated. The result is a deposited Langmuir-Blodgett monolayer film. By successive transversals in opposite directions of the substrate between the water and the air, it is possible to deposit successive monolayers providing on the substrate a Langmuir-Blodgett multilayer film consisting of a multiplicity of monolayers stacked seriatim in a laminar structure. In such structure, the molecules in each monolayer are ideally, as previously described, all oriented to all have the same pointing direction in the thickness of the monolayer. Moreover, it often happens that many or almost all of the molecules in each monolayer on the inside of such structure are coupled head-to-head and tail-to-tail with the molecules in adjacent monolayers on opposite sides of the first-named monolayer. Such coupling produces an ordering in the film of its constituent monolayers in relation to each other. While such a multilayer Langmuir-Blodgett film may in some instances later be modified by processing to more or less eliminate the polar character of its molecules, there is evidence that such mutual ordering of the layers remains to some degree. In any event, the multilayer film as so modified is still referred to as a Langmuir-Blodgett film.
Returning to the Kakimoto et al article, its authors disclose therein (a) the providing of a silicon wafer having evaporated aluminum lines on a surface thereof, (b) the deposition on such surface and over such lines, by the methods described above, of a Langmuir-Blodgett film which may be either monolayer or multilayer, and the initial amphiphilic material of which films consists of a polyamic acid salt, and (c) the subsequent curing of the film to remove the long alkyl chains of the salt and to convert the films into a Langmuir-Blodgett polyimide film. The L-B films so formed varied in number of layers from 1 layer to 120 layers. The electrical characteristics of such films were investigated, and it was concluded that they have insulating characteristics comparable to normal polyimide films.
The Kakimoto et al article fails to disclose anything about the resistance of thin films to moisture, or to recognize any problems that may arise from exposure of such films to moisture or (it inevitably follows) to suggest any solution for such problems. Moreover, there is no disclosure in the Kakimoto et al. article of thin films which are composite in the sense that such films comprise an L-B component film and another component film of different character.